Then there are his friends. They join Little Bill in his adventures and they all have different personalities, which makes their interactions interesting and full of lessons about getting along with others.
His parents are important characters too. They are loving and always there to guide Little Bill. For instance, when Little Bill has a problem at school, his parents give him good advice.
The main character in the 'little bill lavatory story' is surely Little Bill. When a story has a name like this, the first name mentioned is usually the central character. So, Little Bill is the one around whom the events in the lavatory, whatever they may be, revolve.
Obviously, Little Bill himself is a main character. His family members like his mother, father, and grandparents often show up in fanfiction. His friends from the neighborhood or school, such as Fuchsia, also frequently appear as they are important in his daily life.
The 'Beacon Fire Playing with the Dukes' in the title of the novel referred to the protagonist Cao Cao in the Romance of the Three Kingdoms. In the novel, Cao Cao once played a fool with a hero named Zhou Lang because of a misunderstanding, which was why it was called "Beacon Fire Playing with the Dukes".
I'm not sure which novel you're referring to because the name "Who and Who Are My Couples" may refer to many different works. Can you provide more background information or description? This way, I can better answer your questions.